1. Field of the Invention
The present invention generally relates to an optical amplifier and method used in optical communication or optical information processing, and more particularly to an optical amplifier and method for accurately maintaining a uniform level of an amplified light signal.
2. Description of the Related Art
Conventional optical amplifiers which directly amplify light signals use, as an amplifying medium, an optical fiber with a core doped with a rare earth element, and a semiconductor amplifier using a stimulated emission phenomena. Stimulated emission phenomena is well-known, and is related to the emission properties of atoms and molecules at different optical frequencies, and occurs in semiconductors.
When optical amplifiers or similar devices are used for optical transmission, in order to keep the transmission level of a system constant and to stabilize transmission characteristics, the light signal output of an optical amplifier must be stable (e.g., kept constant). Thus, in a conventional optical fiber amplifier, such as that shown in FIG. 6, feedback control is commonly used.
Specifically, the conventional feedback control includes an optical branching unit 20 which is connected to an optical amplifying unit 10. A portion of the light is branched by the branching unit 20 to a light receiving unit 30. The light receiving unit 30 converts the light portion into an electrical signal, thereby to measure the level of the optical signal being output from the optical amplifying unit 10. The output of pumping (e.g., excitation) light from a pumping light source is controlled by a control circuit 51, so that the output is maintained at a constant level.
However, in conventional optical amplifiers, precisely maintaining a constant level of the amplified light signal is difficult. For purposes of this application, "precisely" means keeping a level constant within a certain tolerance range such as for example an amplifier operating with a power input signal Pin of -40 dBm, the tolerance is approximately 15.6 dB, for Pin of -30 dBm, the tolerance is approximately 6.4 dB, and for Pin of -20 dBm, the tolerance is approximately 1.2 dB. Obviously, these values are merely exemplary.
When an input signal power is more than -20 dBm, an effect of ASE power in total amplified signal power is negligible, as shown in FIGS. 7A and 7B. However, when the input signal power is less than -20 dBm, a rate of ASE power in total amplified power increases as input signal power decreases, as shown in FIGS. 7C-7D. For example, at -30 dBm input signal power, a rate of ASE power is approximately 1.4 dB in total amplified signal power (e.g., the output would be equal to P.sub.ASE /P.sub.total =-1.4 dB). Therefore, a gain set by a conventional optical amplifier would be lower (by -1.4 dB) than an original gain.
Moreover, in the conventional systems, even with feedback control, spontaneous emission light distorts the feedback process.
Further, the conventional systems typically include a bandpass filter which fails to maintain an accurate power signal output. In contrast, the invention uses a band rejection filter which is more accurate than a bandpass filter since the band rejection filter subtracts out the ASE light.